1. Field of the Invention
The present invention relates to a discharge tube and more particularly to a discharge tube suitable for stabilizing discharges.
2. Description of the Prior Art
Discharge tubes--in which a discharge gas is sealed into an insulating tube and a voltage is applied between electrodes fitted at the ends of the insulating tube to produce a discharge in the sealed gas--have been in wide use in many fields.
FIG. 4 shows one such conventional discharge tube 1. It has an insulating tube 2 which is a hollow cylindrical member made of such materials as alumina ceramics and drawn inwardly at both ends. The insulating tube 2 is formed by joining a body portion 2a and a cover portion 2b with a glass frit 3. In openings 4 formed at the drawn portions at the ends of the insulating tube 2 are installed a pair of electrodes 5, which are formed by press-forming a perforated thin metal plate into a shape of Rogoski electrode or Harrison electrode that produces a uniform electric field. A base flange portion 5' of each electrode 5 is engaged with the peripheral portion 4' of the opening 4.
Cover-shaped electrode bases 6 made of conductive metal plates are placed on the ends of the insulating tube 2 from outside to cover the openings 4 at which the electrodes 5 are installed. The electrode base 6 clamps between it and the peripheral portion 4' of the opening in the insulating plate 2 the base flange portion 5' of each electrode 5. The end surfaces 6' of the electrode bases 6 are fused by soldering to metalized surfaces 7 formed at the end surfaces of the insulating tube 2 so that the electrodes 5 are securely held by the electrode bases 6, while at the same time sealing the openings 4 where the electrodes 5 are installed.
One of the electrode bases 6 is provided with a supply pipe 8 to supply and seal a discharge gas such as argon under high pressure into the insulating tube 2. The supply pipe 8 is sealed after the discharge gas is introduced.
In such a discharge tube 1, a specified voltage is applied between the electrode bases 6 to generate a uniform electric field in a discharge gap G between the tips of the opposing electrodes 5. A stable discharge occurs in the discharge gap G.
The drawn portions at the ends of the insulating tube 2 elongates the distance along the inner wall surface of the insulating tube 2 from one electrode 5 to the other. This contributes to preventing flashovers along the inner surface of the tube, ensuring that a discharge occurs at a sufficiently high discharge starting voltage in the discharge gap G.
In the above-mentioned conventional discharge tube i however, since the electrodes 5 as a whole project into the inner space of the insulating tube 2 in which a discharge gas is sealed, any conductor with a specified potential, if located near the discharge tube 1, will greatly influence the tube by the electric field of the conductor. This may result in large variations of the discharge starting voltage. That is, stable discharge cannot be obtained due to influences outside the discharge tube.
Because the electrodes 5 are made in the form of Rogoski and Harrison electrodes which are designed to produce a uniform electric field, a precision machining technique is required for machining the surface of the electrodes, making the manufacture very difficult and increasing the cost.
FIG. 5 shows the cross sections of the uniform field generating electrodes of various types. As shown in the figure, although the diameter varies depending on the discharge gap G, a 120.degree. Rogoski electrode must have a diameter about 10 times the discharge gap G and a 90.degree. Rogoski electrode is required to have a diameter about 6.5 times the discharge gap G. Even with the Harrison electrode said to be most suited for generating uniform fields, the electrode diameter necessary is about 5.6 times the discharge gap G. The diameter of the discharge tube 1 as a whole therefore will become excessively large. Conversely, with electrodes with limited diameters, it is only possible to provide a discharge gap G about 1/10 to 1/5.6 the diameter, so, that to obtain a desired discharge starting voltage requires sealing a discharge gas under extremely high pressure. Such electrodes require very precise machining of their contours and even a slight error in contour curvature will result in an uneven field, producing high field intensity areas. This in turn produces localized discharges, making the discharge unstable.
The above discharge tube 1 uses such bonding agents as glass frit 3 to join the body portion 2a and the cover portion 2b to form the insulating tube 2 whose end portions are drawn inwardly. The openings 4 in the body portion 2a and the cover portion 2b where the electrodes 5 are installed are sealed with the electrode bases 6 that are fused to the drawn portion of the insulating tube 2. Because of this construction, the pressure of the high-pressure discharge gas sealed in the insulating tube 2 acts on the electrode bases 6, which seal the openings 4, and also on the cover portion 2b. If the bonding force between the body portion 2a and the cover portion 2b is not large enough to resist this pressure, the bonded portion may break leaking the gas. In that case a desired discharge characteristic is not obtained.